1. Field of the Invention
The present invention relates to a semiconductor photonic devices formed of high-purity molybdenum oxide which emit or absorb light with a short wavelength.
More particularly, the present invention relates to new light emitting diodes which emit blue light and have possibility to overcome problems accompanying to devices made up of known semiconductors such as gallium nitride (GaN) or silicon carbide (SiC). Moreover, the invention relates to photonic devices which emit light with a wavelength shorter than 361 nm in which GaN light-emitting diodes can emit or selectively absorb light having a wavelength shorter than 361 nm.
2. Related Background Art
Light emitting diodes which emit blue light have developed recently in order to realize three primary colors of light and to obtain light with a shorter wavelength for digital video disc (DVD). Developed blue-light emitting devices use gallium nitride (GaN) as an active region which is very important to emit light. The bandgap of GaN is about 3.43 eV which corresponds to a wavelength of 361 nm. Although blue light can be obtained from GaN devices, there are some difficult problems. At first, bulk crystal of GaN has not been obtained because an equilibrium vapor pressure of nitrogen is very high relative to that of gallium. Therefore, substrates made up of sapphire or silicon carbide (SiC) are used. GaN cannot be formed directly on a sapphire substrate because there is lattice mismatch of 16% between sapphire and GaN. Therefore a buffer layer of aluminum nitride (AlN) is formed on a sapphire substrate before growth of GaN. AlN is resistive because it is difficult to dope impurities into AlN. A structure and its fabrication process, therefore, are severely restricted. On the other hand, SiC substrates are very expensive because bulk crystal of SiC can be grown at a very high temperature of 2200-2400° C.
Zinc oxide (ZnO) has possibility to be used to form a blue-light emitting device. However, its bandgap is 3.2 eV which corresponds to a light wavelength of 387 nm which is larger than that GaN devices emit. Moreover, ZnO has many problems to be solved to realize practical devices.
The shortest wavelength of light which semiconductor photonic devices can emit at present is that GaN devices can emit. The maximum density of DVD memory is decided by the wavelength. Therefore, a new photonic device which can emit light with a shorter wavelength is expected in order to increase the maximum density of DVD memory or to replace gas lasers such as He—Cd laser. In addition, a new blue-light emitting device made up of new material is expected because present blue-light emitting devices have many problems as described above. Moreover, a new device which can emit light with a wavelength shorter than 361 nm which GaN devices can emit or a shorter wavelength of deep ultraviolet rays such as 250-350 nm is expected.
The problem to be solved to realize a new device is to obtain a new substrate which replaces expensive substrate such as sapphire or SiC.
The second problem is to realize new semiconductor which can be grown at a lower temperature at which GaN or SiC layers are formed. Large energy is necessary to form semiconductor layers at a high temperature. In addition, there are possibilities that atoms move between layers and a composition is disturbed or dopants move near the interface between layers. It is necessary to form layers of GaN or SiC at a temperature higher than 1000° C.